The invention relates to an apparatus for the galvanic deposition of a metal layer on a substrate.
An apparatus of this kind is used, for example, for the galvanoplastic production of compression tools or molds, especially ones made of nickel. These compression tools are used for the compression molding or injection molding of disks, such as compact disks (CDs), laser vision disks and other information-carrying disks. The above-mentioned molds, which include original molds such as those known as a "glass master" as well as reproductions thereof, are intermediate molds for producing the compression tools. The surfaces of the molds carry information in the form of a relief or recess. The surface structure is transferred from the glass master to the compression tool by means of galvanoplastic reproduction. The information contained in this surface structure is imprinted onto the surface of a plastic material when using the compression tool for injection molding or compression molding. In optical disks, such as compact disks, the relief structure modulates the light of a laser beam permitting the information imprinted on the surface of the disk to be read.
To produce the compression tools or the molds, a metal layer, usually comprising nickel, is deposited on a substrate. The substrate is either an insulating substrate, like glass, having a thin electrically conductive layer, or a metal substrate, like nickel. In either case, the substrate surface has a relief-like structure containing the information to be read. The smallest information unit, called a "pit", is arranged in information tracks and has a spatial wave-length in the micrometer range. The distance between adjacent information tracks is also in the micrometer range. Since the substrate surface may contain several thousand million (10.sup.9) information units, and these corresponding fine structures in the micrometer range have to be transferred to the metal layer, the galvanic metal deposition process has to meet very high standards. The deposited metal layer should be extremely small-grained and free of tension and the thickness of the deposited layer should be relatively large. For example, a compact disk compression tool produced by metal deposition should have a thickness of 295 .mu.m .+-.5 .mu.m. The deposition process should also be carried out at a high speed. Moreover, the apparatus for galvanic deposition should be small in size and simple in its operation.
Another important requirement when creating galvanoplastic metal layers on a substrate is that the thickness of the deposition layer should be uniform across the entire substrate surface. The thickness of the layer should vary only within very close limits. If these limits are not met, the optical disks produced by this metal layer will be of a lesser quality.
Variation in the thickness of the deposited metal layer is dependent on the distribution of the electric current lines between the anode and the substrate, which serves as a cathode. Electric power is usually supplied to the cathode via the shaft of the substrate holder which holds the substrate on a supporting plate. The shaft includes an outer insulating layer to insulate it from the electrically conductive electrolyte. The insulation ensures that the metal ions are deposited only on the substrate surface. To coat different substrates, the supporting plate has to be detached from the shaft. In conventional substrate holders, the seal between the shaft and the substrate holder leaks after frequent use. The leaks may permit the electrolyte fluid to reach locations connected in an electrically conductive manner to the cathode potential. In these locations, metal deposition occurs due to the galvanic process. This deposition is called wild growth. Wild growth disturbs the current line distribution between the anode and the cathode, causes difficulties when detaching the supporting plate from the shaft, and also reduces the layer thickness on the substrate surface.
It is the object of the invention to provide an apparatus for the galvanic deposition of a metal layer whose thickness varies only slightly across the substrate area, and wherein the substrate holder is kept essentially free from wild growth and is easily detachable.